1.ab=cos3x/2cosx/2-sin3x/2sinx/2=cos(3x/2+x/2)=cos2x=1/2
2x=π/3+2kπ
x=π/6+kπ k=0,1,2,.
2.|a|=√(cos²3x/2+sin²3x/2)=1;同理 |b|=1
f(x)=(a-b)²
=a²-2ab+b²
=2-2cos2x
f(x)单调递增cos2x就要单调递减
2kπ≤2x≤π+2kπ
kπ≤x≤π/2+kπ k=0,1,2.k是。。。。